标准ML类型中的问号是什么意思？

例如:

vagrant@precise32:/vagrant$ rlwrap sml
Standard ML of New Jersey v110.76 [built: Mon May 12 17:11:57 2014]
- TextIO.StreamIO.inputLine ;
[autoloading]
[library $SMLNJ-BASIS/basis.cm is stable]
[autoloading done]
val it = fn : ?.TextIO.instream -> (string * ?.TextIO.instream) option
- val s = TextIO.openIn "README.md" ;
val s = - : TextIO.instream
- TextIO.StreamIO.inputLine s ;
stdIn:3.1-3.28 Error: operator and operand don't agree [tycon mismatch]
  operator domain: ?.TextIO.instream
  operand:         TextIO.instream
  in expression:
    TextIO.StreamIO.inputLine s
- 

我知道由于值限制而创建的虚拟类型变量将在其中包含问号,例如

- [] @ [];
stdIn:17.1-17.8 Warning: type vars not generalized because of
   value restriction are instantiated to dummy types (X1,X2,...)
val it = [] : ?.X1 list

...但这不适用于上面的示例,因为不涉及值限制.

在这些讲义中,我发现了以下评论,第23页:

In fact, as indicated by the question marks ? in the error
message, it now has a type that cannot even be named anymore,
since the new but identically named definition of mylist shadows
it.

但这是指一个类型检查错误,无论如何我的TextIO.StreamIO示例不适用,因为没有任何阴影.

编辑添加

所以我想出了我的实际问题,即如何?.TextIO.instream从文件名中获取,但我仍然不知道问号是什么:

vagrant@precise32:/vagrant$ rlwrap sml
Standard ML of New Jersey v110.76 [built: Mon May 12 17:11:57 2014]
val fromFile : string -> TextIO.StreamIO.instream =
=     TextIO.getInstream o TextIO.openIn ;
[autoloading]
[library $SMLNJ-BASIS/basis.cm is stable]
[autoloading done]
val fromFile = fn : string -> ?.TextIO.instream
- TextIO.getInstream ;
val it = fn : TextIO.instream -> ?.TextIO.instream
- TextIO.StreamIO.input1 (fromFile "README.md") ;
val it = SOME (#"#",-) : (TextIO.StreamIO.elem * ?.TextIO.instream) option
- 

第二次编辑

我发现Poly/ML在打印类型时不使用这些问号,所以我认为这是SML/NJ特有的:

Poly/ML 5.5.1 Release
> TextIO.StreamIO.inputLine ;
val it = fn:
   TextIO.StreamIO.instream -> (string * TextIO.StreamIO.instream) option
> val fromFile : string -> TextIO.StreamIO.instream =
    TextIO.getInstream o TextIO.openIn ;
# val fromFile = fn: string -> TextIO.StreamIO.instream
> TextIO.getInstream ;
val it = fn: TextIO.instream -> TextIO.StreamIO.instream
> 

如果有人知道在什么情况下SML/NJ打印出这些问题标记以及它们背后的故事,我仍然很好奇......